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Publié le : 11/5/2013 -Format: Document en format HTML protégé

Human Disease.
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Human Disease.
I

INTRODUCTION

Human Disease, in medicine, any harmful change that interferes with the normal appearance, structure, or function of the body or any of its parts. Since time
immemorial, disease has played a role in the history of societies. It has affected--and been affected by--economic conditions, wars, and natural disasters. Indeed, the
impact of disease can be far greater than better-known calamities. An epidemic of influenza that swept the globe in 1918 killed from 20 million to 50 million people--two
to five times more deaths than were caused by World War I (1914-1918). Within a few months, more than 500,000 Americans died of the 1918 influenza known as the
Spanish flu.
Diseases have diverse causes, which can be classified into two broad groups: infectious and noninfectious. Infectious diseases can spread from one person to another
and are caused by microscopic organisms that invade the body. Noninfectious diseases are not communicated from person to person and do not have, or are not known
to involve, infectious agents. Some diseases, such as the common cold, are acute, coming on suddenly and lasting for no more than a few weeks. Other diseases, such
as arthritis, are chronic, persisting for months or years, or recurring frequently.
Every disease has certain characteristic effects on the body. Some of these effects, called symptoms and signs, include fever, inflammation, pain, fatigue, dizziness,
nausea, and rashes, and are readily apparent to the patient. These symptoms offer important clues that help physicians and other health-care professionals make a
diagnosis. Many times, however, the symptoms point to several possible disorders. In those cases, doctors rely on medical tests, such as blood examinations and X
rays, to confirm the diagnosis.
The course of a disease--that is, the path it follows from onset to end--can vary tremendously, depending largely on the individual and the treatment he or she
receives. For example, otherwise healthy people usually recover quickly from a bout of pneumonia if given proper treatment, whereas pneumonia often proves fatal to
people with a weakened immune system and to those who do not receive prompt, effective treatment. Some diseases run a different course depending on the patient's
age. Chicken pox, for instance, is usually mild in childhood but severe in adults. In the United States, only about 5 percent of chicken pox cases occur in people over the
age of 20, but these cases account for 50 percent of all deaths from the disease.
Scientists, public health officials, and other members of the medical community work diligently to try to prevent disease epidemics. The battle is constant and is fought
on many fronts. There have been many victories. Once-devastating diseases such as smallpox and diphtheria have been virtually eradicated, and many other diseases
that once conferred automatic death sentences can now be either cured or controlled. At the same time, however, new killers have emerged. Acquired
immunodeficiency syndrome (AIDS), hantavirus pulmonary syndrome, and avian flu are among at least 30 diseases that have been identified by scientists since the
early 1970s. Other growing challenges, particularly in the affluent societies of industrialized nations, are so-called diseases of choice, such as alcohol abuse, drug abuse,
or obesity, that result from addictive behavior, poor eating habits, or insufficient exercise.
Complicating matters further are societal changes. Increased international travel accelerates the spread of both new and old diseases: A person infected with an unusual
virus on one continent can arrive--with the virus--on another continent in a matter of hours. Ships, planes, and trucks can transport disease-carrying organisms just as
easily. In 1985 tires imported into Texas from Asia carried larvae of the Asian tiger mosquito, which is a carrier of dengue fever and other tropical diseases. Within five
years, Asian tiger mosquitoes were living in 17 states. Changing dietary habits and the availability in local supermarkets of foods from all parts of the world contribute to
an increase in food-borne illnesses. Some researchers worry that growing populations and the resulting crowded living conditions will increase the risk of epidemics.

II

INFECTIOUS DISEASE

Infectious diseases are caused by microscopic organisms commonly called germs. Physicians refer to these disease-causing organisms as pathogens. Pathogens that
infect humans include a wide variety of bacteria, viruses, fungi, protozoans, and parasitic worms. In addition, it has been theorized that some proteins called prions may
cause infectious diseases.

A

Pathogens

Bacteria are microscopic single-celled organisms at least 1 micron long. Most bacteria species are harmless to humans; indeed, many are beneficial (see eubacteria). But
some are pathogens, including those that cause cholera, diphtheria, leprosy, plague, pneumonia, strep throat, tetanus, tuberculosis, and typhoid fever.
Viruses are tens or hundreds of times smaller than bacteria. They are not cellular, but consist of a core of genetic material surrounded by a protective coat of protein.
Viruses are able to survive and reproduce only in the living cells of a host. Once a virus invades a living cell, it directs the cell to make new virus particles. These new
viruses are released into the surrounding tissues, and seek out new cells to infect. The roll call of human diseases caused by viruses includes mumps, measles,
influenza, rabies, hepatitis, poliomyelitis, smallpox, AIDS, and certain types of cancer.
Fungi are a varied group of generally small organisms that get their food from living or dead organic matter. They germinate from reproductive cells called spores,
which often have a thick, resistant outer coat that protects against unfavorable environmental conditions. This enables spores to survive for long periods of time, which
adds to the difficulty of treating fungal infections. Some fungi are external parasites of humans, causing skin conditions such as ringworm, athlete's foot, and jock itch.
Other fungi invade internal tissues; examples include yeast that infect the genital tract and several fungi species that cause a type of pneumonia.
Protozoans are single-celled, animal-like organisms that live in moist environments. Perhaps the most infamous pathogenic protozoans are species of the genus
Plasmodium, which cause malaria, an infectious disease responsible for over ,Q million deaths worldwide each year. Members of the genus Trypanosoma produce
trypanosomiasis, also known as African sleeping sickness, and Chagas' disease. Other protozoans cause giardiasis, leishmaniasis, and toxoplasmosis.
Parasitic flatworms include tapeworms, which live in the intestines of a host organism. They have a ribbon-like body that may be up to 9 m (30 ft) in length, depending
on the species. Hooks and suckers on the head attach a tapeworm to the intestinal wall, and a tough outer coating protects against the host's digestive juices. Another
group of parasitic flatworms is flukes, which are responsible for several serious tropical diseases, most notably schistosomiasis. See Parasite.
Roundworms, or nematodes, are small, tubelike worms that are pointed at both ends. Species that infect human intestines include pinworms, hookworms, threadworms,
and members of the genus Ascaris. Trichinella spiralis can invade human muscle tissue, often from eating infected pork that has been improperly prepared, causing a
disease called trichinosis.
Prions are extremely tiny protein particles found in the brain, nerve, and muscle cells. A controversial theory states that prions cause disease by changing normal
proteins into an abnormal shape. These mutated proteins in turn force other proteins to change shape, leading to destruction of tissue, primarily in the brain. Some
researchers have hypothesized that prions cause transmissible spongiform encephalopathies, a group of rare infectious diseases that includes Creutzfeldt-Jakob disease
in humans, scrapie in sheep, and bovine spongiform encephalopathy (commonly known as mad cow disease) in cattle. Some evidence suggests that prion-related

disease can be transmitted through food infected with mutated proteins.

B

Spread of Infectious Disease

Some pathogens are spread from one person to another by direct contact. They leave the first person through body openings, mucous membranes, and skin wounds,
and they enter the second person through similar channels. For example, the viruses that cause respiratory diseases such as influenza and the common cold are spread
in moisture droplets when an infected person coughs or sneezes. A hand that was used to cover the mouth while coughing contains viruses that may be passed to
doorknobs, so that the next person to touch the doorknob has a chance of picking up the infectious agent. The bacteria that cause some sexually transmitted infections,
including gonorrhea and syphilis, are transmitted during sexual contact.
Other pathogens involve an intermediary carrier, such as an insect. The malarial parasite, for example, spends part of its life cycle in mosquitoes, then enters a person's
bloodstream when the mosquito bites the person. Many pathogens are spread through contaminated food and water. Cholera bacteria, for example, are spread through
food and water contaminated with the excrement of infected people.

C

New Infectious Diseases

In 1978 the United Nation...


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